This invention relates to RFID tags and, more particularly, to communication of information to tags in the context of a network having a plurality of base stations.
Typical applications of RFID tags pertain to a need to determine the locations of items. An RFID tag is attached to the item, and the location of the item with the attached tag is determined by the use of base stations. Two basic RFID system approaches are used: synchronous, and asynchronous.
A synchronous system employs a system clock, and all base stations and tags are synchronized to the system clock. In some applications time is divided into frames, and each frame has two sub-frames. The first sub-frame is divided into K time slots, during each of which a different one of the base stations may broadcast information to the tags within its range; and the second sub-frame is divided into N time slots, during each of which a different one of the tags transmits information, and all base stations listen. N is the expected maximum number of tags that the system will handle, and K is the number of base stations. The timing is achieved by, illustratively, adding an additional time interval during which beacon signals are transmitted by the base stations and are used by the tags for synchronizing themselves to the system clock. That is, at the beginning of each frame there is a beacon time interval, and the base stations employ this time interval to each send out a synchronizing beacon. This is illustrated by the FIG. 1 timing diagram for an arrangement where N=8 and K=4.
An asynchronous system is one where the tags attempt to send information periodically, but there is no synchronization between the tags and other elements of the system.
Thus, regardless of whether a synchronous system is implemented, or an asynchronous system is implemented, the result is the same in the sense that each tag repetitively transmits information about itself, and the transmitted signal is received by one or more (it is hoped) of the base-stations. Based on the received signal power, or on time-of-flight information, the network of base stations determines, or rather estimates, the location of the tag (and, consequently, the item to which the tag is attached). It may be noted that in a co-pending application, titled “Dual Antenna Base Station for Improved RFID Localization,” and filed on even date herewith, the base stations use directional antenna arrangements that allow for easy determination of tag location by using simple comparison operations.
In a variant of the above approach a tag “listens” to synchronized beacon signals from the different base stations and, using the incoming power information of the received signals in conjunction with the known base station locations, the tag estimates its location (by use of a triangulation technique) and transmits this estimate.
One of the problems of prior art RFID networks is that they lack the ability for the base stations to transmit data that is destined only to a particular tag. The main difficulty stems from the fact that in prior art networks all of the base stations either listen to the tags, or broadcast to all tags; and when they broadcast, they do so other than concurrently, because transmitting concurrently may create “blind spots” that arise from overlapping and destructively combining transmissions.